Method of producing self-assembled cubic FePt nanoparticles and apparatus using same

ABSTRACT

A method comprises: heating a solution of platinum acetylacetonate, Fe(CO) 5 , oleic acid, and oleylamine in dichlorobenzene to a reflux temperature, refluxing the solution, and using the solution to deposit FePt nanoparticles. A magnetic storage medium that includes cubic FePt particles is also provided.

FIELD OF THE INVENTION

This invention relates to methods for making cubic FePt nanoparticles,and to magnetic recording media that includes the FePt nanoparticles.

BACKGROUND OF THE INVENTION

Synthesis and self-assembly of FePt nanoparticles have been of greatinterest to the data storage industry as a possible means for makingnanoparticle-based magnetic media which can enable recording densitiesbeyond 1 Tbit/in². The magnetic bits, which record data in conventionalthin film magnetic media, are comprised of many grains. In order toachieve high recording densities with bits having well-defined shapesand boundaries, it is necessary to reduce the so-called transitionjitter, which is the dominant noise source in today's media. This isusually accomplished by decreasing the size of the grains such that theyremain significantly smaller than the bit size. If the grains become toosmall, however, thermal fluctuations may cause individual grains toswitch randomly, thereby destroying the bit information over time. Toavoid that problem, hard magnetic materials systems such as L1₀ phasesor high anisotropy rare-earth transition metal systems are beingdeveloped, allowing small grains to remain thermally stable. FePt is themost prominent L1₀ candidate material for this approach. See for exampleD. Weller and A. Moser, IEEE Transactions on Magnetics, Vol. 35, pp.4423-4439 (1999). A big limitation of these materials is that typicalsynthesis methods produce a face centered cubic (fcc), lowmagnetocrystalline phase and not the desired high anisotropy L1₀ phase.Thus the films must be subjected to post synthesis annealing to induce aphase transformation from fcc to L1₀.

Self-assembled arrays of FePt L1₀ nanoparticles can, in principle,reduce transition jitter by having uniformly sized magnetic grainsseparated by a well-defined matrix. Jitter is the noise in the magneticsignal from the media that arises from the poorly defined boundary ofthe magnetic bit formed by randomly shaped and randomly sized magneticgrains. Thus the ideal magnetic media would be formed of nanoparticlesthat are of uniform, small size, are packed into arrays with the highestpossible density, and have uniform, e.g., perpendicular magneticorientation.

Chemical syntheses have been developed that produce nearly sphericalFePt nanoparticles that are highly monodispersed with uniform diametersof 3-4 nm. Various methods of chemical synthesis of FePt nanoparticlesare known. These syntheses produce spherical nanoparticles coated withfilms of oleic acid and oleylamine surfactants that are ˜2 nm thick. Oneof the disadvantages of these small spherical nanoparticles is that theyhave low magnetic moment per unit area requiring very sensitive readheads to detect the recorded bits. A typical parameter used to describethe average signal from the media is the M_(r)t. Here M_(r) is theremanent magnetization and t is the thickness of the media. FIGS. 7 and8 show calculations for the M_(r)t for spherical particlesself-assembled on square and hexagonal lattices and cubes self-assembledon a square lattice. Taken into consideration is a region of‘nonmagnetic’ material surrounding the particles with a thickness of‘a’, which may represent surfactant, oxides, other nonmagnetic metalsand/or space. The M_(r)t for all these structures are shown to be˜kM_(r)r³/R² where M_(r) is the remanent magnetization and r and R aregeometrically described in the figure. Notice that the constant kdepends on the shape of the particles and the lattice in which they areplaced. Here k=1.05 for spheres on a square lattice, k=1.21 for sphereson a hexagonal lattice, and k=2 for cubes on a square lattice. Clearlyself-assembled spherical particles give relatively lower M_(r)t ascompared to cubic particles with similar dimensions.

A more critical problem with spherical particles is the fact that themagnetic axes of the particles are difficult to align once deposited ona surface. Others have reported the fabrication of oriented FePtnanoparticles with high coercivity but these nanoparticles werefabricated by electron beam evaporation and grown epitaxially onMgO(001) and NaCl(001) surfaces. Increasing the packing fraction andobtaining alignment of the magnetic axis are both crucial forutilization of chemically synthesized FePt nanoparticles in high densityrecording media.

Synthesis of cubic nanoparticles has been reported in the literature forvarious metals and alloys, including monodispersed silver nanocubes withedge length of 55 nm, cubic PbTe nanoparticles, and cubic PbSenanoparticles for use as quantum dots. The formation of core shell cubicFePt/Fe₃O₄ nanoparticles has been reported, but showed no netorientation or long range self-assembly of these nanoparticles whendeposited on substrates. Chemical synthesis of cubic FePt nanoparticleswas reported by H. Zeng et al. [J. Am. Chem. Soc. 126, 11458 (2004)].

Self-assembled magnetic nanoparticle arrays are described in U.S. Pat.No. 7,041,394 B2, the disclosure of which is hereby incorporated byreference.

It would be desirable to have a magnetic storage media that includesnanoparticles having uniformly oriented magnetic axes and long rangeself-assembly.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a method comprising: heating asolution of platinum acetylacetonate, Fe(CO)₅, oleic acid, andoleylamine in dichlorobenzene to a reflux temperature, refluxing thesolution, and using the solution to deposit FePt nanoparticles.

In another aspect, the invention provides a magnetic storage mediacomprising a substrate, a magnetically soft underlayer on the substrate,and a plurality of generally cubic FePt nanoparticles on themagnetically soft underlayer, wherein the nanoparticles have amagnetization in a direction substantially normal to a surface of themagnetically soft underlayer.

In yet another aspect, the invention provides a magnetic storage mediumcomprising a plurality of cubic FePt nanoparticles arranged in a patternon a magnetically soft underlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are bright-field TEM micrographs of unannealed FePtnanocubes.

FIGS. 3 and 4 are selected area diffraction patterns of unannealed FePtnanocubes.

FIG. 5 is a bright-field TEM micrograph of unannealed FePt nanocubesself-assembled with local square arrays.

FIG. 6 is a cross-sectional view of a magnetic storage medium includingcubic FePt nanoparticles.

FIG. 7 is a schematic representation of a lattice including sphericalparticles.

FIG. 8 is a schematic representation of a lattice including cubicparticles.

FIG. 9 is a schematic representation of particles in a metastable stateand a stable state.

FIG. 10 is a schematic representation of a cubic particle with an L1₀atomic structure in a magnetic field or biasing field.

FIG. 11 is a schematic representation of a cubic particle with an fccatomic structure followed by thermal annealing in a magnetic field orbiasing field.

FIG. 12 is a perspective view of a section of a disc substrate withcubic nanoparticles self-assembled in a locking pattern.

FIG. 13 is a top view of a magnetic recording disc illustrating theformation of magnetic arrays in a grooved locking pattern.

DETAILED DESCRIPTION OF THE INVENTION

Storage media that includes FePt nanocubes may overcome several of theobstacles associated with the development of magnetic data storage mediabased on self-oriented magnetic arrays of FePt nanoparticles.

Chemically synthesized FePt nanoparticles have potential for applicationas magnetic media for data storage. In order to be used as data storagemedia, however, they must be deposited on substrates in such a way thattheir magnetic axes are aligned with respect to one another and withrespect to the substrate, a problem that has yet to be overcome withspherical FePt nanoparticles.

In a first aspect, this invention provides a method for synthesizingsingle crystalline FePt nanoparticles with cubic shapes and with thefacets of the cube parallel to the (001) type crystallographic planes.The synthesis yields particles with shapes that are close to cubic orrectangular. When deposited on surfaces, these particles assemble withlocal order and a square lattice. More importantly, the nanoparticlesorient themselves with their [100] axes perpendicular to the surface onwhich they are positioned, and with local interparticle alignment of the[010] and [001] axes oriented parallel to the surface. This occursbecause the cube is in a stable position when the cube is flat on asubstrate and in a metastable position when the cube is on edge.

The method includes a solution for the chemical synthesis of crystallineFePt nanoparticles with shapes that are close to rectangular and cubic.When deposited onto surfaces from solution, a physical shape anisotropycauses these FePt nanocubes to assemble with oriented crystallographicaxes. The FePt nanocubes range from 4 nm to 10 nm in size and assembleinto arrays that are locally square, and thus have higher M_(r)t valuesthan spherical FePt nanoparticles. Thus, this invention providesprogress in two of the directions critical to the successful developmentof magnetic recording media based on FePt nanoparticles.

In one example of the method, the synthesis of FePt nanoparticles wasperformed under nitrogen in a glove box using iron pentacarbonyl[Fe(CO)₅], and platinum acetylacetonate as sources of Fe and Pt,respectively. In this synthesis Fe generated by decomposition ofFe(CO)₅, is used as a reducing agent for platinum (II) acetylacetonate.A solution of platinum (II) acetylacetonate (0.5 mmol), Fe(CO)₅, (0.5mmol), oleic acid (4 mmol), and oleylamine (4 mmol) in dichlorobenzenewas heated at 4° C. per minute until the reflux temperature of 170° C.was achieved. The heating rate and the reaction temperature are bothsignificantly lower than those used in the standard synthesis withdioctyl ether. The reaction mixture was then refluxed at 170° C. for 24hours. This synthesis generates a colloidal solution of FePtnanoparticles coated with a layer of mixed oleic acid and oleylamine.The colloidal solution was washed using a known standard process toremove the smallest particles. The final solution of FePt nanoparticleswas a 50:50 mixture of hexane and octane with only oleic acid as anexcess surfactant in the solution, where the hexane and octane aresolvents used to redisperse FePt particles after the wash process. Thissolution of nanoparticles was then used to deposit the particles oncarbon-coated transmission electron microscopy (TEM) grids forsubsequent imaging and analysis.

The FePt nanoparticles were self-assembled by placing a drop of thesolution onto carbon-coated copper TEM grids and allowing the solvent todry. The concentration of FePt nanoparticles was approximately 2 mg/ml.TEM imaging and selected area electron diffraction of FePtself-assembled films were performed using a Philips EM420T microscopeoperating at 120 keV. All chemicals used during the process of synthesisand washing of FePt nanop articles were bought from Aldrich and Stremchemicals. Platinum acetylacetonate (99.99%), oleic acid (90%),oleylamine (70%), Fe(CO)₅ (99%), and dichlorobenzene (99%) were used asobtained, without further purification. Anhydrous ethyl alcohol andhexanes were also used without further purification.

The method prepares FePt nanoparticles that self-assemble on surfacesinto textured arrays in which the crystallographic axes of the particlesare oriented with respect to the surface. This has not been achievedusing the spherical FePt nanoparticles produced by the standardsynthesis. This invention modifies the synthesis used by others to formspherical FePt nanoparticles by using dichlorobenzene as the solvent,heating the solution much more slowly and allowing the reaction toproceed at much lower temperatures and for longer times, with the goalof preparing nanoparticles that have cubic shapes that will self-orienttheir crystallographic axes during deposition on surfaces.

FIGS. 1 and 2 are bright-field TEM micrographs of unannealed FePtnanocubes deposited from a non-polar solvent on a carbon-coated copperTEM grid. The cubes are roughly 5 nm on a side. The particles in FIG. 1form a monolayer. The particles in FIG. 2 that are marked in circleshave formed a bilayer. The particles in the first and second layer ofthe bilayer have their sides aligned and one can observe an interferencepattern between the two.

As revealed in the TEM images of FIGS. 1 and 2, the synthesis of thisinvention has successfully produced nanoparticles that are roughly cubicin shape with sides that are about 5 nm in length. Although the shapesand sizes of these particles are not yet uniform, their shapes clearlyapproximate those of cubes and rectangles more closely than those ofspheres. Furthermore, the profiles of the nanoparticles on the TEM gridindicate that they have oriented themselves such that one face of thenanoparticles is in contact with the surface. In other words, if theseparticles are in fact crystalline and have, as one would expect, the fccatomic structure of bulk FePt, then they are oriented with their [100]axes perpendicular to the surface.

FIGS. 3 and 4 show selected area diffraction patterns of unannealed FePtnanocubes obtained with the TEM sample holder tilt angle set at 0°(FIGS. 3) and 20° (FIG. 4). The intense diffraction from the (200) ringsand weak diffraction from the (111) rings indicates a net [100] fibertexture. The anisotropy of the intensity in the rings observed in thediffraction patterns taken at a tilt of 20° also reveals texture in thenanoparticle orientation.

The crystallinity and crystallographic orientation of the FePt nanocubesare revealed by the selected area diffraction (SAD) patterns shown inFIGS. 3 and 4. The diffraction rings clearly arise from the fcc crystalstructure of the FePt lattice. It is important to point out that formagnetic recording applications, one ultimately wants to produce the fctphase rather than the fcc phase. The important feature of the SADpatterns is that the intensities of the rings suggest that the particleshave a net orientation with the [100] axes aligned along the surfacenormal. At 0° tilt this ought to yield intense diffraction from the [0kl] planes perpendicular to the [100] plane. The weak diffractionfeatures from the [111] planes indicates that the order is imperfect.The electron diffraction patterns obtained at a sample tilt angle of 20°reveal anisotropy in intensity around the diffraction rings, alsoindicative of texture in the nanoparticle films. Similar diffractionfeatures have been observed by others for textured films of sputterdeposited FePt and CoPt films.

Close examination of FIG. 2 also reveals several features of theoriented FePt nanocubes. The features highlighted with circles appear tobe two nanocubes stacked on top of one another. Close examinationreveals interference patterns that arise from the superposition of thetwo lattices. These interference patterns appear in a number of suchcases where two particles are stacked on top of one another. Theorientation of these interference features indicates that the [100]planes are oriented parallel to the faces of the nanocubes.

Another interesting feature of the TEM images of FIGS. 1 and 2 is thatlocally the nanocubes assemble themselves into a square array. From thepoint of view of magnetic recording applications, this has the advantageof providing higher packing fraction than the assembly of spheres intoclose packed hexagonal lattices. The nanocubes assemble into a squarearray with a spacing between nanocubes of roughly 3-4 nm. The FePtnanocubes are coated with surfactant films formed by a mixture of oleicacid and oleylamine. The oleyl chain of these surfactants is roughly 2nm in length and so the spacing between particles is consistent withexpectations based on the dimensions of the surfactant chains.

Finally, the nanocube film imaged in FIG. 2 has a mixture of bothmonolayers and bilayers. The interesting feature of the film is that inthe bilayer the nanocubes are stacked on top of one another.Furthermore, the sides of the nanocubes are aligned with one another,indicating that even in the second layer, the [010] and [001] axesretain the same orientation as in the first layer. This suggests that ifallowed to assemble into a three-dimensional structure, these nanocubescould form a cubic lattice with a high degree of interparticlecrystallographic alignment.

FIG. 5 is a bright-field TEM micrograph of unannealed FePt nanocubesself-assembled in local square arrays on a carbon-coated copper TEMgrid. Monolayer and bilayer self-assembly is observed.

In order to be useful in magnetic recording applications, FePt nanocubesmust be prepared as thin films with a fairly uniform distribution acrosslarge areas. Such films could be one or more layers in thickness. FIG. 5shows a large area TEM image of a nanocube film formed by dip coatingonto a TEM grid. As revealed by the image of FIG. 5, these nanocubes canbe deposited with fairly uniform areal density across lengths on theorder of 1 μm. Although the film does not display long range order,within the film, the nanocubes are ordered into a lattice that hasdomains with local square symmetry.

This invention addresses some of the challenges to the development ofmagnetic recording media based on FePt nanoparticles. By synthesizingFePt nanoparticles with roughly cubic shapes, the particles can bedeposited on surfaces such that their crystallographic axes have netalignment with respect to the surface normal.

In the example described above, the invention provides a method thatproduces magnetic FePt nanocubes coated with surfactant. These nanocubesself-assemble on surfaces to form arrays with locally square symmetry.Within the array, the nanocubes are oriented with their [100] axesaligned along the surface normal. The [100] axes lying parallel to thesurface are also aligned with one another. These properties havepotential for application in high density magnetic recording andpatterned media.

In another aspect, the invention provides magnetic storage mediaincluding cubic FePt nanoparticles. FIG. 6 is a cross-sectional view ofa magnetic storage medium 20 constructed in accordance with theinvention. The storage medium includes a substrate 22, a magneticallysoft underlayer 24, and a plurality of substantially cubic FePtnanoparticles 26 on the soft underlayer. The FePt nanoparticles 26 haveeasy axes of magnetization (indicated by arrows) that are substantiallyperpendicular to the plane of the soft underlayer and in the [100]direction of the cubic FePt nanoparticles. The FePt nanoparticles can beas small as 2 nm, while still maintaining magnetic anisotropy andthermal stability. In one example, the FePt nanoparticles have widths inthe range of 4 nm to 10 nm. To reduce the possibility of a coalescenceof the nanoparticles during annealing, a material such as an oxide 28can be positioned between the nanoparticles.

FePt nanoparticles could be placed on top of a magnetically softunderlayer 24 (SUL) (for example, a FeCoB/Ru/FeCoB antiferromagneticallycoupled structure). There is room for a thin interlayer 30, which istypically Ruthenium (Ru). For FePt nanoparticles, the interlayer can beMgO(001). Depending on how thin the surfactant coating is, there couldbe epitaxy between FePt and MgO(001) helping with the perpendicularalignment of the nanocubes.

In addition, there may be some L1₀ order in the as-made state, since thechemical synthesis proceeds at elevated temperature, where a fcc-fctphase transformation onset occurs. The SUL 24 is positioned on thesubstrate 22 that can be, for example, glass, AlMg, etc. An adhesionlayer 23 can be, for example, Ti, Ta, etc. The SUL 24 can be, forexample, 20-200 nm thick FeCoB, NiFe, CoZrNb, CoTaZr, etc. with orwithout an antiferromagnetic coupling layer of 0.5-5 nm Ru. Theinterlayer 30 can be, for example, Ru, MgO, Ta, etc.

FIG. 7 is a schematic representation of a lattice of sphericalparticles. FIG. 8 is a schematic representation of a lattice of cubicparticles. FIGS. 7 and 8 also show the relevant M_(r)t formulas.

FIG. 9 is a schematic representation of a metastable nanoparticle and astable nanoparticle. FIG. 10 is a schematic representation of an FePtnanoparticle that was L1₀ ordered before the self-assembly process. Thebold dotted arrow marked as <001> indicates the tetragonality of theatomically ordered L1₀ phase. FIG. 11 is a schematic showing how anatomically disordered particle can be annealed to transform the particleinto the tetragonal L1₀ structure. The particle on the left has <001>and<100>being similar directions that are related by the cubic atomicsymmetry of the fcc disordered alloy. The particle on the right has beentransformed to L1₀ by the annealing, and the unique easy axis ofmagnetization is indicated by the bold dotted arrow marked as <001>.

The medium can be annealed to produce a phase transition in the FePtnanoparticles from fcc to fct. FIG. 11 illustrates a situation whereFePt is assembled in the fcc disordered atomic phase. Thereforeannealing is required to transform it into the high magnetocrystallineanisotropy phase (atomically ordered L1₀ crystal structure). Thisannealing can be performed in a magnetic field (or another appropriatelyoriented ‘force’) that is substantially perpendicular to the medium tomaintain the easy axis of magnetization in the [001] direction of thecubic FePt nanoparticles.

It may also be possible to form the cubic nanoparticles in the requiredL1₀ atomically ordered state as shown in FIG. 10. If so, other ‘forces’could be used to help orient the nanoparticle (for example, strain,electric field, etc . . . ) during the self-assembly process to alignthe magnetic easy axes. The material between the magnetic cubes in FIG.8 can be, for example a surfactant, an oxide, open space, a nonmagneticmetal, a weakly magnetic material, etc. to prevent agglomeration duringannealing.

This description refers generally to cubic particles. However morespecifically, the facets of the cube need to be parallel to the (100)atomic planes of the FePt. The faceting on the (100) planes can only betrue if the particles are essentially single crystal and faceted on(100).

The nanoparticles can be assembled on a disc that includes a lockingpattern. As shown in FIG. 12, the locking pattern includes a pluralityof pits 40 and lands 42, with a pit depth ranging between 5 nm and 20 nmdeep, depending upon the size the nanoparticles to be used in subsequentprocessing. Using a reactive ion etching process, the bottom surface ofeach pit forming the locking pattern will have a roughness (Ra) equal toapproximately 0.4 to 0.5 nm. Cubic nanoparticles 44 are self-assembledin the pits. Various types of locking patterns may be utilized,including, but not limited to, the square pit pattern of FIG. 12 for aset of one or more bits, a groove pattern (see FIG. 13) for a set of oneor more tracks on a disc 46, or islands in certain locations to create acoarse grid to stabilize the fine grid formed by the nanoparticlesdeposited in the locking pattern.

Additionally, the locking pattern may act as a servo pattern providingpositioning information usable by a magnetic read/write head. Alteringthe sizes and shapes of the pits forming the locking pattern inaccordance with a predefined servo pattern would enable the magneticread/write head to determine its location on the surface of the magneticrecording disc.

While the invention has been described in terms of several examples, itwill be apparent to those skilled in the art that various changes can bemade to the described examples without departing from the scope of theinvention as set forth in the following claims.

1. A method comprising: heating a solution of platinum acetylacetonate,iron pentacarbonyl, oleic acid, and oleylamine in dichlorobenzene to areflux temperature; refluxing the solution; and using the solution todeposit substantially cubic shaped FePt nanoparticles onto a surface,wherein the nanoparticles have facets parallel to (001) crystallineplanes and have [100] axes perpendicular to the surface.
 2. The methodof claim 1, wherein: the solution is heated at 4° C. per minute.
 3. Themethod of claim 1, wherein: the concentration of the platinum (II)acetylacetonate is 0.5 mmol., the concentration of the Fe(CO)₅ is 0.5mmol., the concentration of the oleic acid is 4 mmol., and theconcentration of the oleylamine is 4 mmol.
 4. The method of claim 1,wherein: the reflux temperature is 170° C.
 5. The method of claim 1,wherein: the solution is refluxed for 24 hours.
 6. The method of claim1, wherein: the method forms a colloidal solution of the FePtnanoparticles coated with a layer of mixed oleic acid and oleylamine. 7.The method of claim 6, further comprising: washing the colloidalsolution.
 8. The method of claim 6, further comprising: adding a 50:50mixture of hexane to the washed colloidal solution to redisperse theFePt nanoparticles.
 9. The method of claim 1, further comprising:depositing the FePt nanoparticles in a locking pattern on a substrate.10. The method of claim 1, wherein: the FePt nanoparticles areself-assembled by placing a drop of the refluxed solution ontocarbon-coated copper grids and allowing the solvent to dry.
 11. Themethod of claim 1, wherein: the concentration of the FePt nanoparticleis approximately 2 mg/ml.